Alphafetoprotein (AFP) is a fetal glycoprotein that is associated with the normal growth of the human fetus as with cancerous tumors. For more than three decades, AFP has been used as both a tumor marker and as a birth defect screening agent. More recently, Mizejewski and colleagues have identified a form of AFP in which a normally hidden, growth inhibitory site (GIS) is exposed. This partially denatured, molten glubule form of AFP is likely to play an important role in regulating fetal development and tumor growth. An assay that could rapidly and quantitatively measure this altered form of AFP could be a valuable diagnostic tool. The goal of this project is to develop a sensitive and quantitative assay for the two forms of AFT using microsphere-based flow cytometry.